OSDev.org

[post deleted]

Who wants to leave the GDT behind in the boot sector? For that matter, why be in such a head long rush to get into protected mode?

[post deleted]

mikegonta wrote:

mathematician wrote:Who wants to leave the GDT behind in the boot sector?

This is only a basic GDT, a more sophisticated one can easily be re-established.

mathematician wrote:For that matter, why be in such a head long rush to get into protected mode?

The question is redundant. The fact that this is the 25th anniversary of PM, the proper question to ask is why one would want to remain in Real Mode?
According to The Intel386 SX MicroProcessor Manual, page 6 "Real Mode is required primarily to set up the processor for Protected Mode operation".
Of course I realize that an advanced-extended 32 bit PMode BIOS would make os dev a lot easier.


Mike Gonta
http://groups.google.com/group/aeBIOS/about?hl=en

Um, to do all of the things you might sensibly want to get done before switching to protected mode - such as use the real mode bios (horrors!!) to get a map of memory. Useful if you don't want to plonk your "protected mode bios" straight down on top of somebody else's memory mapped device. In any case, if somebody wants to run their pc in real mode that's fine by me - 25th anniversary of protected mode or not.

The question is redundant. The fact that this is the 25th anniversary of PM, the proper question to ask is why one would want to remain in Real Mode?

i dont think mathematician meant that you shouldnt go into PMode at all, but that you shouldnt go into it so quickly

According to The Intel386 SX MicroProcessor Manual, page 6 "Real Mode is required primarily to set up the processor for Protected Mode operation".

this is in a bootsector -- you cannot 'set up for protected mode operation' properly within the bootsector -- and it makes it very difficult to load the next section of the OS -- plus there are a lot of things you cannot do once you load PMode (the most important is to get the memory map from the BIOS using E820, but there are others as well -- mostly feature detection and initialization)

this is why the normal way to do it, is for your bootsector to load another segment, which is responsible for doing everything that must be done in RMode, and then switch into PMode (this section can be much larger -- mine can expand to, at most, 256KB without requiring me to alter my bootsystem (it is loaded at 0X4 0000 and is allowed to extend to 0X8 0000)



so, the real question is, why are you in such a hurry to enter PMode, that you are willing to make such great sacrifices to do it

if you do it properly, it will turn out much better